1. Field of the Invention
The present invention relates to an injection molding nozzle that incorporates a valve member to control the flow of molding material into a mold cavity. More particularly, the present invention relates to an injection molding nozzle that includes a guide sleeve having inwardly-extending vanes for guiding an axially-movable valve pin so that the pin is properly aligned with a gate.
2. Description of the Related Art
Plastics injection molds often include a flow shut-off valve at the outlet end of a hot runner system to block the flow of molding material between successive injection phases of the machine cycle. A melt flow path originates in the interior of the plastication barrel of an injection molding machine and ends at a xe2x80x9cgatexe2x80x9d into the interior of the mold cavity. A pin-type valve member is often provided in an outlet nozzle adjacent the gate, to control the flow of molten plastic from the nozzle, then through the gate and into the mold cavity. In particular, a portion of the valve pin is movable into the gate to close it after the mold cavity has been filled, to block any further flow of molten material through the gate.
Various arrangements have been proposed to guide the valve pin within the nozzle to align the pin properly with the gate and to prevent the pin from causing damage either to the outer end of the valve pin itself or to the gate. In runnerless mold constructions where the gate provides direct communication between the mold cavity and the injection nozzle, the valve pin enters and closes the gate so that the end of the valve pin serves defines a portion of the mold cavity surface.
Previous valve pin guide arrangements often included support elements disposed in the material flow path, which serve to divide the incoming flow of molten plastic material into several independent streams that recombine at a point downstream, usually immediately adjacent the gate. As a result, the associated molded parts frequently exhibited objectionable flow lines representing areas where the independent flows of plastic had not sufficiently intermixed to join and form a homogeneous stream of melt before entering the mold cavity. For example, in U.S. Pat. No. 5,700,499, entitled xe2x80x9cValve Member Locating Insert For Injection Molding Nozzlexe2x80x9d, which issued on Dec. 23, 1997 to Klaus Bauer, there is disclosed a valve pin guide that includes an outer sleeve and a concentric inner sleeve, wherein the inner sleeve is supported by a radial connecting vane that extends from the outer sleeve to the inner sleeve. In that arrangement, the plastic flow is separated and must combine downstream of the insert.
Another known valve pin guide member that divides the melt flow stream is disclosed in U.S. Pat. No. 5,849,343, entitled xe2x80x9cInjection Molding Apparatus With A One-Piece Gate Insert Locating A Cylindrical Valve Memberxe2x80x9d, which issued on Dec. 15, 1998, to Jobst U. Gellert et al. The guide member disclosed in this patent includes a flow passageway that tapers from an inlet to a narrower outlet that defines the gate. Within the passageway are several axially-extending vanes that extend from the inlet of the insert to the outlet of the insert, which defines the gate, thereby preventing recombination of the several flow streams until the material enters the mold cavity leading to flow lines in the molded part.
Because flow lines in molded parts are objectionable in that they detract from the desired uniform surface appearance of the parts, it is desirable to provide a guide arrangement that avoids the formation of such flow lines while maintaining proper pin alignment. It is therefore an object of the present invention to overcome the shortcomings of the prior art devices and to provide an injection molding nozzle with a valve pin guide that allows the formation of parts that are substantially free of flow lines.
Briefly stated, in accordance with one aspect of the present invention, a nozzle is provided for controlling the flow of molding material from a melt source to an outlet that communicates with a gate that provides a point of entry to a mold cavity. The nozzle includes a nozzle body having a material inlet, a material outlet, and radially-inwardly-extending guide vanes that have guide surfaces. A valve pin is disposed within the nozzle body and is movable in an axial direction, such that with an end of the valve pin is engageable with the gate for selectively opening and closing the gate. More specifically, the valve pin is movable within the nozzle body between a retracted position at which the valve pin is spaced from the gate to allow flow of material through the nozzle, and an extended position, at which the valve pin closes the gate to block the flow of material. The valve pin includes a cylindrical guide section having an outer diameter that allows sliding engagement of the guide section with the guide surfaces of the guide vanes in the nozzle body. A cylindrical valve section of the valve pin is axially aligned with the guide section and includes a forwardmost end having a cross-section that corresponds with that of the gate, so that the valve section defines a closure valve for the gate.
In accordance with another aspect of the present invention, a method is provided for guiding a gate valve closure pin within a tubular nozzle body between an open position and a closed position to allow and block selectively the flow of material through a gate. The method includes moving the valve pin in an axial direction toward the gate from a retracted position to a first internal position without contact of the valve pin with the inwardly-extending guide surfaces associated with the nozzle body. The valve pin is subsequently brought into engagement with the guide surfaces in the nozzle for positive guiding of the valve pin as it continues to move axially toward the gate. In particular, the valve pin is guided by engagement of the valve pin periphery with the guide surfaces of the nozzle body. The gate is ultimately closed as the valve pin is guided to engagement of the forward end of the valve pin with the gate, thereby blocking the flow of material.